1. Field of the Invention
The present invention relates to an alloy and a method for producing magnetic refrigeration material particles using the same.
2. Description of the Related Art
When a magnetic field applied to a certain type of magnetic substance is changed in an adiabatic state, its temperature is changed. This phenomenon is called a magnetocaloric effect. Physically, the degree of freedom of magnetic spins of the magnetic substance is changed by the magnetic field, and the entropy of a magnetic spin system (electron system responsible for (attributed to) magnetism) is changed as a result. With the entropy change, an instantaneous energy transfer occurs between the electron system and a lattice system, resulting in changing the temperature of the magnetic substance. A refrigeration technique using (based on) such a magnetocaloric effect is magnetic refrigeration.
The magnetic refrigeration is expected as environment-conscious (friendly) refrigeration technique because it is chlorofluorocarbon-free and has high energy efficiency. For the magnetic refrigeration in the near room temperature range, an AMR method (Active Magnetic Regenerative Refrigeration) has been proposed as a useful refrigerating method. Besides, a Gd5(Ge, Si)4 based substance, an MnFe(P, As) based substance, an Mn(As, Sb) based substance, an La(Fe, Si)13 based substance and the like have been proposed as materials showing a high magnetocaloric effect in a room temperature range at a low magnetic field.
The La(Fe, Si)13 based substance is promising candidate as a magnetic refrigeration material because it provides a large magnetic entropy change in a low magnetic field and is also substantially free from thermal hysteresis. In a case where the La(Fe, Si)13 based substance is applied to magnetic refrigeration according to the AMR method, it is desirably used by fabricating into spherical particles for practical use (usage). The La(Fe, Si)13 based substance has a problem in a production process of La(Fe, Si)13 phase having an NaZn13 type crystal structure excelling in magnetocaloric effect (which exhibits large magnetic entropy change).
To produce the La(Fe, Si)13 phase, materials such as La, Fe and Si are first prepared at a stoichometric ratio and melted by an arc melting method or a high-frequency melting method. When La and Fe, which are completely non-solid solution systems, are merely undergone a melting process, they are separated into two phases, which are Fe-rich phase and La-rich phase. The former is Fe alloy phase (hereinafter also referred to as α-Fe phase) containing Si and having a bcc crystal structure containing Fe as a main component element. The latter is intermetallic compound phase containing having La as a main component element and Si.
According to a melting process of the arc melting method or the high-frequency melting method, coarse crystal phases of Fe-rich phase and La-rich phase are mutually convoluted and show an intricate metallographic structure. Subsequently, the integrated alloy is subjected to a heat treatment at a temperature of about 900 to 1100° C. for a long period of time to produce gradually La(Fe, Si)13 phase by interdiffusion of the elements. Thus, the production process of the La(Fe, Si)13 phase using a bulk material by applying an ordinary melting method has drawbacks that it is essential to perform the heat treatment at a relatively high temperature for (long term of) several days to several months.
Meanwhile, JP-A 2004-100043 describes that a liquid quenching method is applied to production of a ribbon-like magnetic refrigeration material in order to eliminate the necessity of a long-term heat treatment in an La(Fe, Si)13 phase production process. As described above, since the magnetic refrigeration material is desirably used by fabricating into the spherical particles, the ribbon-like magnetic refrigeration material has a drawback that it has poor practical utility.
JP-A 2004-099928 describes a magnetic refrigeration material containing metalloid elements (B, C and the like). It describes that the addition of the metalloid elements in a range of 1.8 to 5.4 atomic % to the magnetic refrigeration material produces La(Fe, Si)13 phase in 75 volume % or more immediately after casting of a molten alloy. But, fabricability into spherical particles and uniformity of the properties among the particles obtained by fabricating into the spherical particles are not taken into consideration.
To apply the La(Fe, Si)13 based substance to the magnetic refrigeration, it is necessary to fabricate into practical small pieces (spherical particles or the like). To do so, there are a method of subjecting a mother alloy to the heat treatment to produce the La(Fe, Si)13 phase and breaking into small pieces, and a method of breaking a mother alloy into small pieces and subjecting them to the heat treatment to produce the La(Fe, Si)13 phase. The former method has a disadvantage that the filling factor of the magnetic refrigeration material lowers depending on the pulverized shapes because the mother alloy undergone the heat treatment is pulverized into small pieces. There is a problem that cracks (cracking) are produced within the small pieces by a stress applied when pulverizing to make them brittle, and the small pieces are finely divided during the magnetic refrigeration operation to disturb the operation.
As a method of breaking an alloy material (mother alloy) into small pieces by melting, an atomizing method, a rotary disc process (RDP) and a rotary electrode process (REP) are generally known. Spherical particles produced by such a method are subjected to a heat treatment to produce La(Fe, Si)13 phase, so that the spherical particles (magnetic refrigeration material particles) suitable for magnetic refrigeration can be obtained. Especially, the rotary electrode process capable of producing the spherical particles without involving the mother alloy melting process in a crucible is suitable as a method for producing the spherical particles to apply the La(Fe, Si)13 based substance to the magnetic refrigeration. By the rotary electrode process, the particles each close to a spherical shape can be produced efficiently.
However, in a case where the mother alloy produced on the basis of a conventional material composition is applied to the rotary electrode process, the composition ratio of the spherical particles becomes variable because of the coarse two-phase separated state of the mother alloy, and it becomes a cause of degrading the properties of the magnetic refrigeration material particles. When the rotary electrode process is applied to the production of the magnetic refrigeration material particles, raw materials such as La, Fe and Si are prepared at the stoichiometric ratio of La(Fe, Si)13, melted by high-frequency melting or the like, and cast by using a mold to produce the mother alloy of an La(Fe, Si)13 based substance.
The mother alloy produced based on a conventional material composition has a metallographic structure that the coarse Fe-rich phase and La-rich phase exist together. Where this mother alloy is used to produce spherical particles by the rotary electrode process, the composition of each of the spherical particles becomes variable largely because of the coarse two-phase separated state of the mother alloy. Where the spherical particles are subjected to the heat treatment to produce the magnetic refrigeration material particles having the La(Fe, Si)13 phase, there is a difference in generation of the La(Fe, Si)13 phase on the basis of the composition variation of the spherical particles, and property variations of the magnetic refrigeration material particles become large. In addition, the generation efficiency of the La(Fe, Si)13 phase is also degraded because the interdiffusion of the elements is hard to occur in certain compositions.
The magnetic refrigeration material particles (spherical particles) produced by using the conventional mother alloy have variations in Curie temperature Tc because of the composition variations. Where such spherical particles are charged in a container and applied to the magnetic refrigeration according to the AMR method, an optimum operation temperature (close to Tc) also becomes variable in terms of the magnetocaloric effect because of variations of the Curie temperature Tc among the spherical particles. Thus, a sufficient refrigerating effect cannot be obtained by a thermal cycle test according to the AMR method.